1. Field of the Invention
The present invention relates to a plasma display panel, and an image display device using the same, particularly to a plasma display panel, which may be abbreviated to a PDP hereinafter, suitable for making display images highly minute, and an image display device using the same.
2. Description of the Related Prior Art
An AC plane-discharge type PDP is a display device wherein a great number of minute discharge spaces (discharge cells) airtightly closed between two glass panel plate are set up. Referring to a drawing, this AC plane-discharge type PDP will be briefly described hereinafter. FIG. 2 is a perspective exploded view illustrating a part of an ordinary PDP structure. The PDP illustrated in FIG. 2 is a panel wherein a front panel plate 21 made of glass and a back panel plate 28 made of glass are adhered to and integrated with each other, and is a reflection type PDP wherein phosphor layers 32 emitting red (R), green (G) and blue (B) rays are formed on the side of the back panel plate 28.
The front panel plate 21 has a pair of sustaining electrodes, which are also called display electrodes, formed on its face opposite to the back panel plate 28 and in parallel to have regular intervals.
The pair of sustaining electrodes is composed of transparent common electrodes (hereinafter referred to merely as X electrodes) 22-1, 22-2, . . . , and transparent independent electrodes (hereinafter referred to merely as Y electrodes or scanning electrodes) 23-1, 23-2, . . . .
In the X electrodes 22-1, 22-2, . . . , non-transparent X bus electrodes 24-1, 24-2, . . . for compensating for the conductivity of the transparent electrodes are set up to extend in the direction shown by an arrow D2 in FIG. 2, and in the Y electrodes 23-1, 23-2, . . . , Y bus electrodes 25-1, 25-2, . . . are set up to extend in the same direction.
The X electrodes 22-1, 22-2, . . . , the Y electrodes 23-1, 23-2, . . . , the X bus electrodes 24-1, 24-2, . . . , and the Y bus electrodes 25-1, 25-2, . . . are insulated from the discharge spaces, in order to be AC-driven. In other words, these electrodes are covered with a dielectric layer 26 composed of a low melting point glass layer which generally has a thickness of several tens of microns. This dielectric layer 26 is covered with a metal oxide layer 27.
As the metal oxide layer 27, there is generally used a magnesium oxide (MgO) layer formed by EB vapor deposition and having a thickness of about 1 xcexcm. This magnesium oxide layer has a high secondary electron emission factor and excellent resistance against sputtering by ions, and functions so as to cause an improvement in discharge characteristics.
The above-mentioned metal oxide layer is generally called xe2x80x9cprotective layerxe2x80x9d. An example thereof is a single layer composed of a magnesium oxide layer which is directly formed on a display electrode by chemical vapor deposition (CVD), as disclosed in JP-A-10-261362.
The back panel plate 28 has, on its face opposite to the front panel plate 21, address electrodes (hereinafter referred to merely as A electrodes) crossing three-dimensionally and perpendicularly to the X electrodes 22-1, 22-2, . . . , and the Y electrodes 23-1, 23-2, . . . of the front panel plate 21.
The A electrodes 29 are set up to extend in the direction shown by an arrow D1 in FIG. 2. Barrier ribs 31 for separating the A electrodes 29 from each other are set up in order to prevent the expanding of discharge (regulate the region of discharge). The pair of sustaining electrodes composed of the X electrode and the Y electrode may also be separated from each other by means of the barrier rib along the direction shown by the arrow D2. The respective phosphor layers 32 emitting red, green and blue light rays are successively applied in the form of stripes so as to cover groove faces between the barrier ribs 31.
FIG. 3 is a view showing the structure of a main cross section of the PDP, as is viewed along the direction shown by the arrow D2 in FIG. 2, and illustrates a single discharge cell, which is the smallest unit of a cell. In FIG. 2, the boundaries of the discharge cell are roughly shown by broken lines. The inside of a discharge space 33 is filled with a discharge gas (e.g., a mixed rare gas such as helium, neon, argon, krypton or xenon) for generating plasma.
When a voltage is applied between the X display electrodes and the Y display electrodes, plasma 10 is generated by electrolytic dissociation of the discharge gas. FIG. 3 schematically illustrates a situation in which the plasma 10 is generated. Ultraviolet rays from this plasma excite the phosphors 32 to emit fluorescent rays. The fluorescent rays from the phosphors 32 are emitted through the front panel plate 21 outside the discharge cells. The rays emitted from the respective discharge cells constitute images on a display screen.
In the case of attempting to make the PDP highly minute, the gap distance (discharge gap) between the X-Y display electrodes must be made narrow with an improvement in the minuteness of the discharge cells. When the discharge gap is made narrow, the electric field intensity between the electrodes increases. As a result, sputtering is promoted with an increase in ion impact against the protective layer. By the sputtering, the protective layer is stricken off and the dielectric is made naked so that the discharge becomes unstable. Consequently, the panel cannot be driven. In other words, a problem that the lifetime of the panel becomes short arises.
In order to prevent the reduction in the lifetime of the panel, the protective layer should be made thick. According to the prior art, however, as the protective layer is made thicker, a large number of cracks are generated. It is therefore impossible to make the thickness of the protective layer sufficiently thick.
Since the protective layer cannot be easily made thick in the prior art as described above, it is indispensable to form the dielectric layer for insulating the electrodes from discharge. It is difficult to cut off this step of forming the dielectric layer.
Furthermore, with an improvement in the minuteness of the discharge cells (reduction in the cell pitch), the ratio of the luminescent area in the PDP is reduced; therefore, a drop in display brightness thereof is also caused.
Therefore, in order to overcome the above-mentioned problems in the prior art, a first object of the present invention is to provide a plasma display panel wherein a drop in the brightness thereof with an improvement in the minuteness thereof can be prevented and the luminescent efficiency thereof to applied electric power can be improved by making a high-quality and thick protective layer. A second object of the present invention is to provide a plasma display device having this plasma display panel.
In the case that a metal oxide layer such as a MgO layer is formed as a protective layer on a glass panel plate or a dielectric layer, the linear thermal expansion coefficient of this metal oxide layer is generally larger that of the glass panel plate or the dielectric layer as an undercoat. Therefore, with a drop in temperature after the formation of the layer, tensile stress acts on the formed metal oxide layer so that cracks are generated in the metal oxide layer.
The number of the generated cracks becomes larger as the thickness of the metal oxide layer becomes larger. Incidentally, in order to reduce the number of the generated cracks, it is advisable to decrease the difference in linear thermal expansion coefficient between the above-mentioned glass panel plate or dielectric layer and the above-mentioned metal oxide layer. In this way, the metal oxide layer can be made so as to have a larger thickness and higher-quality.
Therefore, the above-mentioned object of the present invention can be attained in the way that a protective layer covering a display electrode set on a transparent panel plate, such as a glass panel plate, constituting a front panel plate of a plasma display panel (hereinafter referred to as a transparent front panel plate is made of: a bi-layered metal oxide layer composed of a first metal oxide layer for decreasing the difference in linear thermal expansion coefficient and a second metal oxide layer covering the first metal oxide layer.
More specifically, the first metal oxide layer desirably comprises a metal oxide polycrystal layer which has a larger linear thermal expansion coefficient than a transparent front panel plate or a dielectric layer and which is, for example, made of MgO or made mainly of MgO. The second metal oxide layer desirably comprises a metal oxide polycrystal layer which has a larger secondary electron emission coefficient than the transparent front panel plate or the dielectric layer, which has a larger linear thermal expansion coefficient than the first metal oxide layer, and which is, for example, made of at least one selected from CeO2, CaO and TiO2 or made mainly of at least one selected from the same group.
It is allowable that the first and second metal oxide layers contain an inevitable amount of an impurity which is naturally incorporated. Correspondingly to this fact, the above-mentioned wording xe2x80x9cmade mainly ofxe2x80x9d is used.
A dielectric layer may be set between the metal oxide layer and the display electrode on the transparent front panel plate. According to the present invention, however, the protective layer can be made thick as described above. It is therefore sufficient that only the above-mentioned bi-layered metal oxide layer is directly formed on the display electrode without forming any dielectric layer. In this case, the step of forming the above-mentioned dielectric layer can be cut off. Thus, costs for the process for producing the PDP can be reduced.
About the protective layer composed of the bi-layered metal oxide layer of the present invention, it is desired that the total layer thickness of the first metal oxide layer and the second metal oxide layer is set to at least 2 xcexcm. In the case of a protective layer composed of a single layer of MgO in the prior art, a problem that the number of cracks increases from 15 or 16 to several tens if the thickness thereof is set to 2 xcexcm or more. However, according to the present invention, when the total layer thickness of the bi-layered metal oxide layer is from 2 to 5 xcexcm, cracks are not generated at all. When the layer thickness is set to 10 to 40 xcexcm, only about 3 to 9 cracks are generated. The number of the generated cracks is remarkably reduced. Thus, the PDP of the present invention can be sufficiently put to practical use.